Method for manufacturing a part coated with a protective coating

ABSTRACT

A method for manufacturing a part coated with a protective coating includes: forming a protective coating across all or part of the surface of a part, wherein the part includes a refractory alloy including a niobium matrix containing metal silicide inclusions, wherein the protective coating is formed by a pack carburization method from a cement including: i. a mixture A of (Nb x Ti 1-x ) 3 M 3 CrSi 6  and M 0.6 Cr 0.4 Si where M denotes Fe, Co or Ni and x is between 0 and 1, or ii. a mixture B of M′Si, NbSi 2  and Nb 4 M′ 4 Si 7  where M′ denotes Fe, Co or Ni.

BACKGROUND OF THE INVENTION

The invention relates to parts coated with a protective coating, and tomethods of fabricating such parts.

At present, within the hottest parts in turbine engines, onlysuperalloys based on nickel are used on an industrial scale. Althoughsuch nickel-based superalloys are coated with a thermal barrier system,their utilization temperature may be limited to 1150° C. because of theproximity of their melting point.

Recent research work has focused on using new materials based onrefractory metals capable of being used at temperatures higher than theutilization temperatures of nickel-based superalloys. These families ofmaterials are commonly referred to as: refractory metal matrix compositematerials.

Among the solutions that have been found, niobium-based alloys seem tobe particularly promising for replacing or being used in addition toexisting nickel-based superalloys. These various alloys have theadvantage of presenting melting points that are higher than those ofexisting superalloys. Furthermore, niobium-based alloys can alsoadvantageously present densities that are relatively low (6.5 grams percubic centimeter (g/cm³) to 7 g/cm³, in comparison with 8 g/cm³ to 9g/cm³ for nickel-based superalloys). Such alloys can thus advantageouslyserve to reduce significantly the weight of turbine engine parts, e.g.the high pressure turbine blade sets, because of their low density andbecause of their mechanical properties that are close to the mechanicalproperties of nickel-based superalloys at temperatures close to 1100° C.

Such niobium-based alloys may generally comprise numerous additionalloying elements such as silicon (Si), titanium (Ti), chromium (Cr),aluminum (Al), hafnium (Hf), molybdenum (Mo), or tin (Sn), for example.These alloys present a microstructure constituted by a niobium matrix(Nb_(ss)) reinforced by addition elements dissolved in solid solution.This phase provides the alloys with acceptable toughness at lowtemperature. The refractory matrix is associated with intermetallicprecipitates, often refractory metal silicides of composition andstructure that may vary depending on the addition elements (M₃Si,M₅Si₃).

In spite of the large amounts of progress that have been made concerningthe composition of alloys seeking to increase their ability to withstandoxidation at high temperature, this ability may not suffice forenvisaging a direct industrial application.

There therefore exists a need to have novel materials presenting bothgood mechanical properties (concerning toughness when cold and creep athigh temperature for moving parts) and also good resistance to corrosionand to oxidation at very high temperature.

There also exists a need to have novel methods enabling such materialsto be obtained.

OBJECT AND SUMMARY OF THE INVENTION

In a first aspect, the present invention provides a method offabricating a part coated with a protective coating and comprising thefollowing step:

-   -   forming a protective coating over all or part of the surface of        the part, the part comprising a refractory alloy having a        niobium matrix with inclusions of metal silicides present        therein, the protective coating being formed by a pack        cementation process using a cement comprising:        -   i) a mixture A of (Nb_(x)Ti_(1-x))₃M₃CrSi₆ and of            M_(0.6)Cr_(0.4)Si, where M designates Fe, Co, or Ni and x            lies in the range 0 to 1; or        -   ii) a mixture B of M′Si, of NbSi₂, and of Nb₄M′₄Si₇, where            M′ designates Fe, Co, or Ni.

Below, the term “mixture A” designates a mixture of(Nb_(x)Ti_(1-x))₃M₃CrSi₆ and of M_(0.6)Cr_(0.4)Si, where M designatesFe, Co, or Ni, and x lies in the range 0 to 1.

Below, the term “mixture B” designates a mixture of M′Si, of NbSi₂, andof Nb₄M′₄Si₇, where M′ designates Fe, Co, or Ni.

The present invention serves advantageously to form protective coatingon niobium-based parts, which coatings are capable of developingprotective oxide layers in order to improve the ability of the parts towithstand oxidation without that affecting their mechanical properties.The pack cementation process serves advantageously to treat parts ofcomplex shape, to deposit thereon a plurality of elements in a singlestep, and to obtain a protective coating of thickness that is uniform.

The method of the invention lies on selecting particular mixtures ofdonor alloys for the cement that is used (above-mentioned mixtures A andB). These mixtures serve advantageously to obtain protective coatingsthat improve the ability of treated parts to withstand oxidation verysignificantly.

The part may be constituted by a refractory alloy comprising a niobiummatrix having metallic silicide precipitates present therein.

The protective coating that is formed may comprise a plurality ofdistinct phases, these phases possibly being present in the form ofmutually superposed layers. The formation of these phases is governed bythe solid state interdiffusion of the deposited elements and of theelements constituting the substrate.

When the cement comprises a mixture B of FeSi, of NbSi₂, and ofNb₄Fe₄Si₇, the protective coating that is formed comprises Nb, Fe, andSi. In particular, under such circumstances, a plurality of phases ofthe protective coating may each comprise Nb, Fe, and Si.

In a variant, when the cement comprises a mixture B of CoSi, of NbSi₂,and of Nb₄Co₄Si₇, the protective coating that is formed comprises Nb,Co, and Si. In particular, under such circumstances, a plurality ofphases of the protective coating may each comprise Nb, Co, and Si.

In another variant, when the cement comprises a mixture B of NiSi, ofNbSi₂, and of Nb₄Ni₄Si₇, the protective coating that is formed comprisesNb, Ni, and Si. In particular, under such circumstances, a plurality ofphases of the protective coating may each comprise Nb, Ni, and Si.

In an embodiment, the cement may comprise a mixture B, and before thebeginning of the pack cementation process:

-   -   the ratio [weight of M′Si in the cement]/[total weight of the        mixture B in the cement] may preferably be in the range 5% to        30%; and    -   the ratio [weight of NbSi₂ in the cement]/[total weight of the        mixture B in the cement] may preferably be in the range 5% to        30%; and    -   the ratio [weight of Nb₄M′₄Si₇ in the cement]/[total weight of        the mixture B in the cement] may preferably be in the range 50%        to 80%.

The term “total weight of the mixture B in the cement” should beunderstood as the following sum: [weight of M′Si in the cement] +[weightof NbSi₂ in the cement] +[weight of Nb₄M′₄Si₇ in the cement].

In an implementation, when the cement comprises a mixture B with M′=Feand before the beginning of the pack cementation process:

-   -   the ratio [weight of M′Si in the cement]/[total weight of the        mixture B in the cement] may lie in the range 5% to 30%, for        example in the range 10% to 20%; and    -   the ratio [weight of NbSi₂ in the cement]/[total weight of the        mixture B in the cement] may lie in the range 5% to 30%, for        example in the range 10% to 20%; and    -   the ratio [weight of Nb₄M′₄Si₇ in the cement]/[total weight of        the mixture B in the cement] may lie in the range 50% to 80%.

Preferably, the cement may comprise a mixture A.

Using such a mixture advantageously makes it possible to obtain aprotective coating presenting very good resistance against oxidation andagainst corrosion while hot. Using such a mixture makes it possible toform a protective coating comprising Nb, Cr, and Si. In particular,under such circumstances, a plurality of phases of the protectivecoating may each comprise Nb, Cr, and Si.

Preferably, the cement may comprise a mixture A and M may designate Coor Ni, and in particularly preferred manner M designates Ni.

In particular, the cement may comprise a mixture A and x may be notequal to 0, in particular x may be equal to 1. In such circumstances,the cement comprises a mixture A of Nb₃M₃CrSi₆ and of M_(0.6)Cr_(0.4)Si,where M designates Fe, Co, or Ni.

Preferably, the cement may comprise a mixture A and x=0. In other words,the cement may comprise a mixture A of Ti₃M₃CrSi₆ and ofM_(0.6)Cr_(0.4)Si, where M designates Fe, Co, or Ni.

In addition to the above-mentioned advantages in terms of ability towithstand oxidation and corrosion while hot, using such a mixture makesit possible advantageously to obtain a protective coating that presentsvery good compatibility with the underlying part, and consequently verygood adhesion thereon.

Such a mixture makes it possible to obtain a protective coating compriseNb, Ti, and Si. In particular, under such circumstances, a plurality ofphases of the protective coating, or indeed all of them, may eachcomprising Nb, Ti, and Si.

The presence of Ti within the protective coating makes it possibleadvantageously to obtain a very good match between the expansioncoefficients of the underlying part and of the protective coating.Furthermore, the presence of Ti in the mixture A can modify thedirection in which the protective coating grows and can serve to limitthe evaporation at high temperature of any Ti contained in the part(which evaporation can occur by volatile titanium halides forming whilethe protective coating is being formed).

In an implementation, when the cement comprises a mixture A ofTi₃Fe₃CrSi₆ and of Fe_(0.6)Cr_(0.4)Si, the protective coating that isformed comprises Nb, Ti, Cr, Si, and Fe. In particular, under suchcircumstances, a plurality of phases of the protective coating, andpossibly all of them, may each comprise Nb, Ti, Cr, Si, and Fe.

In a variant, when the cement comprises a mixture A of Ti₃Co₃CrSi₆ andof Co_(0.6)Cr_(0.4)Si, the protective coating that is formed comprisesNb, Ti, Cr, Si, and Co. In particular, under such circumstances, aplurality of phases of the protective coating, or indeed all of them,may each comprise Nb, Ti, Cr, Si, and Co.

In another variant, when the cement comprises a mixture A of Ti₃Ni₃CrSi₆and of Ni_(0.6)Cr_(0.4)Si, the protective coating that is formedcomprises Nb, Ti, Cr, Si, and Ni. In particular, under suchcircumstances, a plurality of phases of the protective coating, orindeed all of them, may each comprise Nb, Ti, Cr, Si, and Ni.

Preferably, the cement may comprise a mixture A and the ratio [weight of(Nb_(x)Ti_(1-x))₃M₃CrSi₆ in the cement before the beginning of the packcementation process]/[weight of M_(0.6)Cr_(0.4)Si in the cement beforethe beginning of the pack cementation process] may lie in the range 0.9to 1.1.

By way of example, the niobium matrix may include inclusions of Nb₅Si₃and/or inclusions of Nb₃Si.

Preferably, the part may be subjected to a temperature lying in therange 1100° C. to 1300° C. throughout all or part of the step of formingthe protective coating.

Preferably, the duration of the pack cementation process may lie in therange 1 hour (h) to 100 h, e.g. in the range 2 h to 96 h.

In an implementation, the thickness of the protective coating that isformed may be greater than or equal to 15 micrometers (μm), e.g. may liein the range 15 μm to 50 μm.

It may be advantageous to limit the thickness of the protective coatingto a value that is less than or equal to 50 μm in order to avoidobtaining a fragile coating with the associated problems of excessivespalling during thermal cycling. It can also be very advantageous tomake a coating of thickness that is greater than or equal to 15 μm inorder to obtain satisfactory protection against oxidation.

The invention also provides a part comprising a refractory alloycomprising a niobium matrix having metal silicide inclusions presenttherein, the surface of the part being coated by a protective coating,the protective coating comprising a phase having the followingstoichiometry (atomic proportions):

-   -   (Nb_(x)Ti_(1-x))₃M_(β)Cr_(γ)Si_(δ)X_(ε) where M designates Fe,        Co, or Ni, X designates one or more other elements that might be        present, x lies in the range 0 to 1, 6 lies in the range 5 to        8.5, and the sum β+γ lies in the range 3 to 7; or    -   Nb₄M′_(η)Si_(θ)X′_(ε′) where M′ designates Fe, Co, or Ni, X′        designates one or more other elements that might be present, η        lies in the range 3.2 to 4.8, and θ lies in the range 6 to 8.

As described above β is greater than zero.

Such a coated part can be obtained by performing a method as describedabove. When a method as described above is performed to obtain thecoated part of the invention the use of a mixture A leads to aprotective coating comprising a phase of(Nb_(x)Ti_(1-x))₃M_(β)Cr_(γ)Si_(δ)X_(ε), and the use of a mixture Bleads to a protective coating comprising a phase ofNb₄M′_(η)Si_(θ)X′_(ε′).

The coated parts of the invention present very good resistance againstoxidation and corrosion when hot. The protective coating is preferablysuch that the phase having the stoichiometry specified above is presenton the outside surface of the protective coating, said outside surfacebeing situated on the side of the coating furthest from the surface ofthe coated part.

X and X′ may be at least one of Al and/or of Hf. In an embodiment, ε andε′ may be less than or equal to 1, e.g. 0.5, e.g. 0.3.

When the protective coating comprises a phase of(Nb_(x)Ti_(1-x))₃M_(β)Cr_(γ)Si_(δ)X_(ε), the silicon may be present inthis phase at an atomic content lying in the range 44% to 48%. When theprotective coating comprises a phase of Nb₄M′_(η)Si_(θ)X′_(ε′), thesilicon may be present in this phase at an atomic content lying in therange 45% to 49%.

In following Table 1, various examples are given by way of illustrationof stoichiometry for phases of the protective coating that can beobtained in the context of the invention.

TABLE 1 M or M' = Fe M or M' = Co M or M' = Ni Mixture Nb_(21.3)Fe₂₄Nb_(21.3)Co_(18.9) Nb_(22.1)Ni_(19.7) A Cr_(8.9)Si_(45.8)Cr_(13.9)Si_(45.9) Cr_(13.5)Si_(44.6) Mixture Nb_(25.7)Fe_(27.1)Nb_(26.7)Co_(26.3) Nb_(27.2) B Si₄₇ Si_(46.7) Ni_(26.6)Si_(45.9)

Preferably, M may designate Co or Ni. In particularly preferred manner,M designates Ni.

In an embodiment, the protective coating may have thickness greater thanor equal 15 μm, e.g. lying in the range 15 μm to 50 μm.

In an embodiment, the part may be a turbine engine blade. In a variant,the part is to form an integral portion of a combustion chamber or toform a turbine ring or nozzle.

The present invention also provides a turbine engine including a part asdefined above.

The present invention also provides an aircraft including a turbineengine as defined above.

BRIEF DESCRIPTION OF THE DRAWINGS

Other characteristics and advantages of the invention appear from thefollowing description of the accompanying drawings, in which:

FIG. 1 is a diagrammatic and fragmentary section of a part of theinvention;

FIG. 2 is a diagrammatic and fragmentary view of a reactor suitable forperforming a method of the invention;

FIG. 3 illustrates in simplified manner the reaction scheme for forminga protective coating in the context of a method of the invention;

FIGS. 4A to 4C are photographs of different protective coatings obtainedon the surfaces of Nb_(ss)—Nb₅Si₃ alloys by performing an implementationof the method of the invention;

FIGS. 5A to 5C are photographs of other protective coatings obtained onthe surfaces of Nb—Si alloys by performing a variant of the method ofthe invention; and

FIG. 6 shows the results of cyclic oxidation tests at 1100° C. on Nb—Siparts coated with protective coatings of the invention in comparisonwith an Nb—Si part that is not coated.

DETAILED DESCRIPTION OF IMPLEMENTATIONS

FIG. 1 is a section of a part 1 coated with a protective coating 2. Theprotective coating 2 is formed on the surface of the part 1, whichcomprises a niobium matrix having metallic silicide inclusions presenttherein.

The thickness e of the protective coating 2 that is formed may lie inthe range 15 μm to 50 μm, for example. The thickness e of the protectivecoating 2 corresponds to its greatest dimension measured perpendicularlyto the surface S of the part 1.

FIG. 2 shows a reactor suitable for use in the context of a method ofthe invention. As shown, the reactor is in the form of an enclosure 10in which the part 1 for treatment is present. The part 1 is present in acement 11 that comprises firstly a mixture of donor alloys 13 andsecondly an activator agent 12. As shown, the part 1 is in contact withthe cement 11. The composition of the mixture of donor alloys 13 isselected as a function of the protective coating that is to be obtainedon the part 1. The donor alloy mixture 13 may be a mixture A or amixture B, where these mixtures are defined above. By way of example,the activator agent 12 may be selected from: SiCl₄, SiF₄, NH₄Cl, NH₄F,metallic halides such as metallic fluorides or chlorides, e.g. CrCl₃,and mixtures thereof. The donor alloys 13 and the activator agent 12 areboth in powder form.

The activator agent 12 may be present in the cement before beginning ofthe pack cementation process, at a content by weight lying in the range0.5% to 2% of the total weight of the mixture of donor alloys 13 in thecement. The pack cementation is performed in the enclosure 10.

The cement 11 also comprises an inert diluant, e.g. comprising silica(SiO₂) and/or alumina (Al₂O₃), e.g. in the form of a mixture of Al₂O₃and SiO₂. The inert diluant advantageously serves to avoid particles ofcement agglomerating at the surface of the zone to be coated while thetemperature of the ingredients is being raised. The inert diluant may bepresent in the form of powder in the cement before the beginning of thepack cementation process.

The weight of inert diluant in the cement before the beginning of thepack cementation process may lie in the range 0.8 times to 1.2 times thetotal weight of the donor alloy mixture 13 in the cement, and may forexample be substantially equal to the total weight of the donor alloymixture 13 in the cement.

In order to initiate the method of the invention, the enclosure 10 israised to a temperature lying in the range 1100° C. to 1300° C., forexample. Throughout all or part of the method of the invention, theenclosure 10, by way of example, be filled with an inert gas or may besubjected to a primary or secondary vacuum. In a first step 20, a metalhalide is formed from a metal forming part of the donor alloys and ahalide coming from the activator agent. The metal halide as formed inthis way then diffuses in the gas phase to the part 1 that is to betreated (step 21) onto which it becomes adsorbed (step 22). Thereafter,the metal halide decomposes, i.e. the metal and the halide separate(step 23). The metal is deposited on the surface of the part 1 and cansubsequently diffuse within it (step 24) and the halide returns to thegas phase. On contact with the donor alloys, the halide diffusing in thegas phase (step 25) can once more form a metal halide and reinitiate theabove-described cycle of forming the protective coating.

EXAMPLES

Unless specified to the contrary, the compositions of the protectivecoating phases given below are given in atomic proportions.

Example 1

Each of FIGS. 4A to 4C is a photograph of a part coated with aprotective coating obtained by performing a method of the invention.

In this example, the protective coatings are formed by using thefollowing mixtures B (the proportions are by weight):

-   -   FIG. 4A: 20% FeSi+20% NbSi₂+60% Nb₄Fe₄Si₇;    -   FIG. 4B: 10% CoSi+10% NbSi₂+80% Nb₄Co₄Si₇;    -   FIG. 4C: 20% NiSi+20% NbSi₂+60% Nb₄Ni₄Si₇.

For these three types of coating, the part and the cement are maintainedat a temperature of 1200° C. throughout the pack cementation process,and the duration of the pack cementation process is 24 h. As shown inthis figure, the protective coatings that are formed comprise aplurality of distinct phases. These various phases are in the form of astack and they are superposed on one another.

In FIG. 4B, the phase marked “1” corresponds toNb_(27.7)Co_(26.9)Si_(45.9), the phase marked “2” corresponds toNb_(60.8)Co_(0.9)Si_(38.2), and the phase marked “3” corresponds toNb_(62.7)Si_(36.7).

In FIG. 4C, the phase marked “1” corresponds toNb_(32.6)Ni_(0.2)Si_(67.3), the phase marked “2” corresponds toNb_(29.3)Ni_(25.3)Si_(45.4), the phase marked “3” corresponds toNb_(40.3)Ni_(19.7)Si_(39.9,) and the phase marked “4” corresponds toNb_(62.7)Si_(37.2).

Example 2

Each of FIGS. 5A to 5C is a photograph of a part coated with aprotective coating obtained by performing a method of the invention.

In this example, the protective coatings are formed by using mixtures Aas set out in Table 2 below (in the column “donor alloys”). The contentsthat appear for the donor alloys are contents by weight. The chemicalnature of the phases obtained in the protective coating are specified inthe “analyzed phase probe composition” column.

TABLE 2 Metal- lographic section (SEM- back- scattered electron image:“BSE Donor Analyzed phase probe composition mode”) alloys (% at) shownin (1) 1) Nb₀, ₆Ti₁₆, ₁Hf₀Cr₁₅, ₉Al₀Si₄₆Fe_(20, 8) FIG. 5A 50% M₇Si₆- 2)Nb₁₈, ₄Ti₁₁, ₆Hf₆Cr₃, ₂Al₀, ₂Si₄₃, ₄Fe_(16, 5) TiFe + 3) Nb₃₂, ₃Ti₁₇,₂Hf₃, ₁Cr₁, ₁Al₀, ₁Si₄₄, ₄Fe_(1, 9) 50% B20Fe 4) Nb₂₆, ₅Ti₁₇, ₈Hf₉,₁Cr₀, ₇Al₀, ₁Si₄₅, ₁Fe_(0, 6) (2) 1) Nb₁₀, ₅Ti₉, ₉Hf₂, ₈Cr₁₂, ₇Al₀Si₄₆,₆Co_(17, 1) FIG. 5B 50% M₇Si₆- 2) Nb₁₃, ₃T₁₉, ₃Hf₁Cr₁₂, ₆Al₀Si₄₆,₂Co_(17, 2) TiCo + 50% 3) Nb₁₇, ₃Ti₇, ₇Hf₁, ₆Cr₂, ₉Al₀Si₄₆, ₇Co_(23, 5)B20Co 4) Nb₁₄, ₂Ti₉, ₁Hf₅, ₁Cr₃, ₄Al₀Si₄₆, ₂Co_(21, 6) 5) Nb₃₁,₈Ti₁₇Hf₂, ₇Cr₂, ₆Al₀Si₄₁, ₉Co_(3, 9) 6) Nb₂₃, ₅Ti₁₉, ₉Hf₁₀, ₁Cr₀,₅Al₀Si₄₅, ₇Co_(0, 3) (3) 1) Nb₁₁, ₁Ti₁₃, ₁Hf₀, ₈Cr₂₇, ₇Al₀Si₄₄,₇Ni_(2, 4) FIG. 5C 50% M₇Si₆- 2) Nb₁₇, ₁Ti₁₀, ₁Hf₁, ₉Cr₁₂,₅Al₀Si₄₄Ni_(14, 1) TiNi + 50% 3) Nb₁₄Ti₁₀, ₇Hf₄, ₁Cr₁₁, ₉Al₀Si₄₄,₄Ni_(14, 7) B20Ni 4) Nb₂₆, ₁Ti₁₂, ₄Hf₁, ₉Cr₃, ₈Al₀Si₄₂, ₉Ni_(12, 8) 5)Nb₃₂Ti₁₇, ₂Hf₂, ₆Cr₂Al₀, ₂Si₄₁, ₃Ni_(4, 8) 6) Nb₂₂, ₈Ti₂₀Hf₁₀, ₄Cr₀,₅Al₀, ₂Si₄₆Ni_(0, 2) For Table 2 above: (1) M₇Si₆ − TiFe = Ti₃Fe₃CrSi₆and B20Fe = Fe_(0.6)Cr_(0.4)Si (2) M₇Si₆ − TiCo = Ti₃Co₃CrSi₆ and B20Co= Co_(0.6)Cr_(0.4)Si (3) M₇Si₆ − TiNi = Ti₃Ni₃CrSi₆ and B20Ni =Ni_(0.6)Cr_(0.4)Si For these three types of coating, the duration of thepack cementation process is 24 h and the part and the cement aremaintained at 1200° C. throughout the pack cementation process. Thecoated part is a material and silicide composite (MASC) alloy asdescribed in patent U.S. Pat. No. 5,942,055, and having the followingcomposition in atomic percentages: Nb = 47%; Ti = 25%; Hf = 8%; Cr = 2%;Al = 2%; and Si = 16%.

Example 3 Cyclic Oxidation Tests at 1100° C. on coated Nb—Nb₅Si₃ Parts

The protection against oxidation conferred by the protective coatingshas been evaluated. The coatings were obtained under the same conditionsas for Example 2. The results are given in FIG. 6.

The lifetimes of parts protected by these coatings were improvedcompared with bare parts (i.e. without coating). When M═Co, thelifetimes of protected parts were multiplied by a factor of at least 15compared with M═Fe, and when M═Ni, the lifetimes of protected parts weremultiplied by a factor of 30 compared with M═Fe. As can be seen in FIG.6, the tests were duplicated for each of the coatings in order todemonstrate the reproducible nature of the results obtained.

The highest performance coatings withstand approximately 3000 oxidationcycles at 1100° C. In cyclic conditions, these coatings present goodresistance to oxidation up to 1200° C.

It can be seen that the non-coated alloy becomes oxidized very quicklyand in very significant manner (large increase in mass as a result ofoxidation). If contact with the oxidizing medium is sufficientlyprolonged, the oxides that are formed subsequently spall off, therebyleading to a reduction in weight, as can be seen for the curves plottedin FIG. 6.

Furthermore, the formation of these protective coatings on niobium-basedparts can advantageously serve to divide by 200 the increases in weightrecorded during isothermal exposure at 1100° C. And in isothermalconditions, these coatings can advantageously confer effectiveprotection up to 1300° C.

The term “comprising/containing a” should be understood as“comprising/containing at least one”.

The term “lying in the range . . . to . . . ” should be understood asincluding the boundaries.

1. A method of fabricating a part coated with a protective coating andcomprising: forming a protective coating over all or part of a surfaceof the part, the part comprising a refractory alloy having a niobiummatrix with inclusions of metal silicides present therein, theprotective coating being formed by a pack cementation process using acement comprising: i) a mixture A of (Nb_(x)Ti_(1-x))₃M₃CrSi₆ and ofM_(0.6)Cr_(0.4)Si, where M designates Fe, Co, or Ni and x lies in therange 0 to 1; or ii) a mixture B of M′Si, of NbSi₂, and of Nb₄M′₄Si₇,where M′ designates Fe, Co, or Ni.
 2. A method according to claim 1,wherein the cement comprises a mixture A and wherein M designates Co orNi.
 3. A method according to claim 2, wherein M designates Ni.
 4. Amethod according to claim 1, wherein the cement comprises a mixture Aand wherein x=0.
 5. A method according to claim 1, wherein the cementcomprises a mixture A and wherein x is not equal to
 0. 6. A methodaccording to claim 5, wherein x=1,
 7. A method according to claim 1,wherein the cement comprises a mixture A and wherein the ratio [weightof (Nb_(x)Ti_(1-x))₃M₃CrSi₆ in the cement before the beginning of thepack cementation process]/[weight of M_(0.6)Cr_(0.4)Si in the cementbefore the beginning of the pack cementation process] lies in the range0.9 to 1.1.
 8. A method according to claim 1, wherein the cementcomprises a mixture B and wherein before the beginning of the packcementation process: the ratio [weight of M′Si in the cement]/[totalweight of the mixture B in the cement] lies in the range 5% to 30%; andthe ratio [weight of NbSi₂ in the cement]/[total weight of the mixture Bin the cement] lies in the range 5% to 30%; and the ratio [weight ofNb₄M′₄Si₇ in the cement]/[total weight of the mixture B in the cement]lies in the range 50% to 80%.
 9. A method according to claim 1, whereinthe part is subjected to a temperature lying in the range 1100° C. to1300° C. throughout all or part of the step of forming the protectivecoating.
 10. A method according to claim 1, wherein a thickness of theprotective coating formed lies in the range 15 μm to 50 μm.
 11. A partcomprising a refractory alloy comprising a niobium matrix having metalsilicide inclusions present therein, the surface of the part beingcoated by a protective coating, the protective coating comprising aphase having the following stoichiometry:(Nb_(x)Ti_(1-x))₃M_(β)Cr_(γ)Si_(δ)X_(ε) where M designates Fe, Co, orNi, X designates one or more other elements that might be present, xlies in the range 0 to 1, 6 lies in the range 5 to 8.5, and the sum β+γlies in the range 3 to 7; or Nb₄M′_(η)Si_(θ)X′_(ε) where M′ designatesFe, Co, or Ni, X′ designates one or more other elements that might bepresent, η lies in the range 3.2 to 4.8, and 0 lies in the range 6 to 8.12. A turbine engine including a part according to claim
 11. 13. Anaircraft including a turbine engine according to claim 12.